1. Field of the Invention
The present invention relates generally to earth boring drill bits and more particularly to such drill bits having diamond cutting elements.
2. Description of the Related Art
Known drill bits include diamond bits which can be fabricated from either natural or synthetic diamonds. Conventional diamond drill bits utilize a number of different types of diamond cutting elements, for example, polycrystalline diamond compact (PDC) cutters, thermally stable diamond product (TSP) cutters, mosaic-type cutters, and natural and impregnated diamond.
PDC diamond cutting elements can be made by forming an amalgam of crystalline diamond and cobalt which is sintered into disc shapes. Such discs are then bonded, usually by a diamond press, to a tungsten carbide slug. The slug cutters are then attached by drill bit manufacturers to a tungsten carbide slug or stud which is fixed within a driIl bit body designed by the bit manufacturer.
TSP cutters are PDC diamond cutting elements from which metallic elements are leached. Some types of TSP cutters replace the interstices from which the cobalt carbide is leached with another element, such as silicone, which has a thermal coefficient of expansion similar to the diamond. TSP cutters may be used to form a mosaic cutter in which a plurality of geometrically-shaped TSP elements are arranged and bonded in a desired shape to form a unitary cutting surface. They also may be used as individual cutters.
Prior art PDC cutting elements degenerate dramatically above a temperature of about 700.degree.-750.degree. Centigrade due to the difference in thermal coefficient of expansion between the diamond and the tungsten carbide. This prevents utilizing high melting-point alloys to bond the PDC cutting element to a carbide slug and also prevents direct infiltration of a PDC cutting element, either by itself or in combination with a slug carrier or stud, into a bit formed by infiltration in a high temperature furnace. Temperatures for forming such bits are typically 1100.degree. Centigrade and above. It would be desirable to provide an artificial diamond having a high resistance to thermal degradation.
U.S. Pat. No. 4,976,324 issued Dec. 11, 1990 to Tibbitts for a drill bit having diamond film cutting surface discloses a bit which includes a cutting element having a PDC diamond substrate which is coated with a vapor deposition diamond film. The PDC element is generally mounted on a supporting member of tungsten carbide which in turn is braised or sintered to a carrier member on the bit body. Also disclosed therein is a TSP cutting element having a diamond film thereon with the TSP element being bonded to a supporting member on the bit body in a known manner.
While the diamond film in the above-captioned Tibbitts patent provide a cutting face having a lower porosity, which is desirable from the standpoint of wear and impact resistance, the PDC elements on which the diamond layer is formed prevent the use of high temperatures in brazing, bonding or infiltration processes for securing the cutting elements to the bit and/or to carrier members which are in turn secured to the bit. While the TSP cutting elements can be subjected to higher temperature than the PDC cutting elements, pure crystalline diamond, such as that created by chemical vapor deposition, has better resistance to temperature and lower porosity. Also, a diamond layer created by chemical vapor deposition can be formed to create a shaped, i.e., non-planar, cutting face.
It would be desirable to provide a drill bit including synthetic diamond cutting elements which are made completely of diamond produced by chemical vapor deposition.